The present invention relates to the field of geology and geophysics. In particular the invention relates to the estimation of characteristics such as effective stresses and pore pressure from seismic data.
Many subsurface-related human activities, like oil and gas exploration and production, mining, underground construction, and earthquake prediction, can benefit from direct estimates of the state of stress of the Earth subsurface. The importance of stress estimates increases when principal stresses are not equal to each other (non-hydrostatic stress state) and there appear some preferred directions in geological media (like directions of maximum and minimum stresses). Planning of drilling operation and mine construction are examples of applications requiring good knowledge of existing stressed state or pore fluid pressure. In this case poor estimates of effective stresses may lead to additional costs and safety problems related to geological hazards and instability of borehole or mine. Furthermore, fracture networks providing major routes for fluid flow control the development of many existing oil fields and orientation of fractures is usually controlled by direction of maximum horizontal stress. Therefore stress characterization performed prior to production may reduce risk in reservoir management decisions.
Elastic waves propagating through stressed solids contain information about applied effective stresses. These waves are usually of small amplitudes so they do not alter the investigated material. In general, velocities of propagated waves depend on unstressed elastic moduli, effective stress magnitudes and nonlinear elastic parameters. In principle, velocities of these waves may be inverted for orientations, magnitudes of principal total stresses and pore pressure.
First, such techniques were applied in non-destructive testing of materials. See e.g. Guz"", A. N., Makhort, F. G., Guscha, O. I., Lebedev, V. K., 1974, Foundations of ultrasonic non-destructive method of measuring stressed state in solids: Kiev, Naukova Dumka. Easy implementation of these techniques was due to the following conditions:
A priori knowledge (measurement) of elastic moduli and nonlinear parameters of material,
Unrestricted access of sources and receivers (usually transmission along direction of one of the principal stresses),
Application to one or two-dimensional state of stress.
Dealing with the Earth subsurface all these assumptions are, generally, invalid. For example, in oil and gas exploration the parameters of subsurface rocks are unknown, measurements are restricted to sources and receivers all located at the surface or in borehole and stressed state is triaxial.
The use of elastic (ultrasonic) waves applied for measuring uniaxial and biaxial in situ stress in mines of Kola Peninsula has been described. See: Bakulin, V. N., and Protosenya, A. G., 1981, Ultrasonic polarizational method of determination of stress in rock mass: Collection of papers: Mining Geophysics, Tbilisi, 96-97; and Bakulin, V. N., and Protosenya, A. G., 1982, Nonlinear effects in propagation of elastic waves through rocks: Proc. USSR Acad.Sc. (Dokl.Akad.Nauk SSSR), 2, 314-316.
This technique was later updated to estimate triaxial stresses. See, Bakulin, A. V., and Bakulin, V. N., 1992, Method for determining rock mass burst danger: Patent of USSR No. 1786273. Limitations of these techniques include the requirement of two or more boreholes (transmission configuration) drilled in the formation and the assumption that all formation properties are a priori known.
Using surface shear-wave seismic or shear-wave VSP data for predicting subsurface stress regimes has been suggested. See e.g., Winterstein, D. F., 1991, Method of layer stripping to predict subsurface stress regime: U.S. Pat. No. 5,060,203. This method uses two orthogonally polarized sources of shear waves and many multicomponent receivers either at the surface or along a borehole. Layer-stripping technique allowed obtaining orientation of maximum and minimum horizontal principal stresses in each layer from polarization directions of two split shear waves at near-vertical incidence. The maximum stress is assumed along the polarization direction of fast shear wave whereas the minimum is orthogonal to it (or along the polarization direction of slow shear wave). This technique suffers from limitations including:
The method gives neither magnitudes of effective principal stresses nor differences between them.
The method uses orthogonally polarized shear wave vibrators which are expensive and rarely used modification of seismic measurements,
The method does not apply to conventional PP and PS reflection data routinely used in the geophysical industry,
One of principal stresses is a priori assumed vertical everywhere in the geological media. Although in many cases it is true, the method does not allow to check whether this assumption is valid or not,
In reservoirs with multiple fracture sets (for example containing two sets of vertical non-orthogonal sets of fractures) shear wave polarizations would not be aligned with directions of principal stresses. The proposed measurements do not allow the revelation of such situations.
Some other models oriented towards extracting pore pressure from estimate of vertical effective stress are reported in the literature (see, Bowers, G., 1995, Pore pressure estimation from velocity data: accounting for overpressure mechanisms besides undercompaction: SPE Drilling and Completion, 10, 89-95; and Eaton, B. A., 1975, The equation for geopressure prediction from well logs: SPE, 5544, 1-11), but they typically disregard non-hydrostatic nature of the Earth stress field, i.e. assume that all principal stresses are equal to each other. The latter assumption is often violated.
Another method has been disclosed for estimating the in situ stress magnitudes of the formations using sonic borehole tool. See, Sinha, B. K., 1998, Method for estimating formation in-situ stress magnitudes using a sonic borehole tool: U.S. Pat. No. 5,838,633. This method utilizes multi-frequency inversion of sonic velocities for estimating orientation and magnitude of effective stress as well as nonlinear rock properties. However, this method can have the following drawbacks:
It uses a borehole drilled into the formation where stresses are to be determined,
It is designed primarily for biaxial effective stress or the difference between the maximum and minimum stress and does not estimate triaxial state of stress,
It gives local estimates of effective stress field in a relatively small volume around borehole (3-4 well diameters),
It does not allow obtaining 3D distribution of stress field unless very dense surface system of boreholes is drilled.
At least in part, therefore, the present invention aims to alleviate or avoid some of the aforementioned problems.
According to the invention a method is provided for estimating stress characteristics from seismic data. The method includes receiving seismic data acquired using a plurality of sensors adapted to sense seismic energy, the seismic data representing subterranean characteristics within a subterranean region, receiving properties of rock at a location within the subterranean region, and estimating one or more stress characteristics for at least a sub-region in the region by combining the seismic data and the rock properties using a relationship between the stress characteristics in the sub-region and elastic stiffness and/or sonic velocity in the sub-region.
The relationship is preferably based on an effective medium theory, and even more preferably based on a non-linear elasticity theory.
The method also preferably includes the steps of analyzing in the seismic data azimuth and offset dependence of seismic signatures for seismic anisotropy thereby determining a set of anisotropic coefficients; identifying directions of minimum, intermediate and maximum stresses from orientation of principal axes of seismic anisotropy and signs of the anisotropic coefficients; and inverting the anisotropic coefficients thereby estimating magnitudes of principal stresses in the sub-region using non-linear elastic constants and vertical total stress information derived from the rock properties.
According to the invention a method is also provided for estimating pore pressure from seismic data.
The invention is also embodied in a system for estimating stress characteristics, and in a computer readable medium that is capable of causing a computer to estimate stress characteristics from seismic data.